Recently, work has begun on an attempt to isolate nucleolar sequence-specific DNA-binding proteins from rat tumor cells, in the hope that such proteins will have some function in controlling the transcription of preribosomal RNA. The isolation of such proteins would be greatly facilitated by the use of ribosomal DNA-cellulose columns, rather than nucleolar DNA-cellulose, as originally proposed. In this project, rat ribosomal DNA (rDNA) will be purified to near-homogeneity and inserted into a bacterial plasmid vector for amplification. Clones containing rDNA will be selected by colony hybridization and grown to the desired quantities. All federal guidelines for biohazard safety in recombinant DNA research will be observed. The purified rat rDNA will be used to test the hypothesis that specific rDNA-binding proteins exist in the cells of interest. If so, rDNA-cellulose columns will be used to isolate such proteins. They will be characterized by two-dimensional polyacrylamide gel electrophoresis, DNA-binding assays, and effects on the in vitro transcription of preribosomal RNA.